Singulation and scribing are well-known processes in the semiconductor industry, in which a cutting machine is used to work a workpiece or substrate such as a semiconductor wafer, which could for example comprise silicon but is not so limited. Throughout this specification, the term “wafer” is used to encompass all these products. In a singulation process (also referred to as dicing, severing, cleaving for example), a wafer is completely cut through such as to singulate the wafer into individual dies. In a scribing process (also referred to as grooving, scoring, gouging or furrowing for example), a channel or groove is cut into a wafer. Other processes may be applied subsequently, for example full singulation by using a physical saw along the cut channels. Alternatively or additionally, holes may be formed in a wafer using a drilling process. Throughout the present specification, the term “cutting” will be used to encompass singulation, scribing and drilling.
However, the overall semiconductor technology trend in miniaturization is to decrease the thickness of the wafer, and as wafer thicknesses decrease, it has been shown that laser technology becomes more advantageous for singulation than the use of a mechanical saw. Exploiting high-power lasers for such material processing has significant advantages in comparison with mechanical counterparts such as, for instance, drilling and sawing, and laser processing has a great versatility in coping with small and delicate workpieces.
Laser removal of the semiconductor material occurs due to a rapid temperature increase of a relatively small area in which the laser beam is focused, which causes local material to melt, explosively boil, evaporate and ablate. Laser singulation has challenging requirements, including the delicate balance between the process throughput and the workpiece (die) quality. The quality and throughput of the process are determined by laser parameters such as fluence, pulse width, repetition rate, polarisation as well as distribution of the polarisation, wavefront shape and its phase modification and wavelength. It has been proposed to use a multiple beam laser cutting approach, for example in WO 1997/029509 A1, wherein a linear cluster of focused laser beams, which may be arranged in a linear array of laser spots, is used to ablate substrate material along a scribe-line, thus causing the substrate to be radiatively scored along the line of ablation. The use of multiple beams in this manner as opposed to a single (more powerful) beam may provide various advantages, in particular a reduction in the defect density created during the cutting process.
One of the quantitative assessments of the laser process quality is the die or wafer fracture strength, which determines a tensile stress at which the wafer breaks. Uniaxial flexure tests are commonly employed for the determination of fracture strength for brittle materials and have been adopted for wafer strength measurements. These tests include three- and four-point bending tests, which are commonly used to measure fracture strength.
It is believed that the fracture strength of the laser-separated wafers depends on the level of laser-induced defects such as micro-cracks and chip-outs, which appear after the laser singulation process in the wafer. These defects are generated by a high stress at the interface between the bulk semiconductor material and the local laser-processed area. The high stress is produced by high temperature gradients between the bulk and processed zones by acoustic shock waves emerging during the process and chemical transformations of the process side walls of the die. The region of the semiconductor material which contains such defects is commonly referred to as the “heat-affected zone”. The fracture strength is typically different for the front and back sides of the wafer, and indeed there are techniques, processes and wafer layouts which can result in significantly different back-side and top-side strengths.
Recent advances in ultrashort pulse (“USP”) lasers enable wafer processing to be performed more delicately, since the temporal pulse widths of those lasers are shorter than the typical times of electron-phonon relaxation in solids, which is responsible for heat transfer from photo-excited electrons to the lattice, the pulse width being less than 1-10 ps depending on the particular material being processed. USP lasers can provide an improvement to the die strength of the material, however the productivity of wafer processing systems using such USP lasers is reduced due to numerous reasons, including for example a lesser heat diffusion-induced interaction volume.
To increase the laser machining speed and hence productivity, an arrangement of laser pulses has been proposed in which there are two repetition periods or “bursts” of laser pulses emitted by a laser source, such that the time between consecutive pulses t1 is shorter than the time between consecutive bursts t2, i.e. the time between the first pulse p1 of consecutive bursts, but longer than the laser pulse width Δτ. The burst is an integer number n of laser pulses pn grouped within period t2.
Dicing with bursts of ultrashort laser pulses at repetition frequencies (i.e. 1/t1) in the order of tens of MHz (from about 10,000 to about 90,000 kHz) has been shown to lead to more efficient material removal compared with a process using individual laser pulses. However, the heat load generated with pulses with these repetition frequencies induces a reduction in die strength when compared with use of a single pulse.
It has been shown that the burst mode arrangement can provide an increase in productivity. However, this increase leads to a decrease of the die strength of the singulated semiconductor dies, to such an extent that dies produced using this method may not comply with current market demands. It has been shown that the die strength substantially decreases because of an incubation effect: a cumulative effect of accumulation of damage and temperature in material.
The present invention seeks to provide an improved laser cutting method, which provides both improved wafer or die strength and also increased productivity.
In accordance with the present invention this aim is achieved by implementing a burst sequence of laser pulses, having a significantly high pulse repetition frequency.
The use of a burst mode having pulse repetition frequencies in the range of 0.1 GHz (100 MHz) to 5000 GHz (5 THz) further assists the material removal efficiency with a so-called “ablation cooling” effect, in which relatively hot material is ablated before the onset of effective heat transfer from the molten material region to the bulk by a thermal diffusion, causing the net temperature of the material to decrease, or at least not to increase substantially. It has been found that dicing of semiconductor wafers with bursts of laser pulses with pulse repetition frequencies in this range provides increases in both productivity (i.e. higher material removal efficiency) and die strength. The latter stems from two factors:
i) The energy of single pulses in each burst has to be scaled down such that the total energy of the individual pulses in the burst is equal to a certain optimum single pulse energy, so that the intensities of resulting laser-induced shock-waves, generated upon laser pulse impinging into the target, are dramatically suppressed. Shock-waves are one of the sources of defect formation and as a result reduction of die strength.
ii) Heat diffusion is suppressed by the ablation cooling mechanism, so that the size of the heat-affected zone, one of the crucial factors affecting the die strength, is decreased. In more detail, the heat load to the target is reduced by efficiently converting the energy of the train of laser pulses during the burst into material ablation, rather than creating shock waves and undesired thermal transport to the bulk of the material. This reduction in heat load in turn reduces the re-solidified laser-irradiated zone, which minimises the generation of material structural defects and crack formation.
Furthermore, such a methodology permits flexibility, for example in the use of polarization and multibeam arrangements, beam shaping as well as energy tuning of individual single pulses (burst shaping). Such flexibility allows the process to be optimally tailored for specific purposes.
The productivity of the laser dicing process and die strength of semiconductor devices are therefore increased when burst mode pulse repetition frequencies between 100 MHz and 5 THz are used, as compared with MHz and kHz burst mode pulse repetition frequencies.